Clipping circuit and image processing device employing such a clipping circuit

ABSTRACT

In an image processing device, an edge detecting circuit  1  produces edge signals that represent edge components present in image signals, and an adder circuit  2  superimposes the edge signals on the image signals to produce edge-enhanced image signals. The image signals before edge enhancement are fed to a range setting circuit  3  so that, based on the image signals preceding and following a target image signal that is about to be processed by a clipper  4 , a range in which the data value of the target image signal is allowed to vary is set. Based on the range of data values thus set by the range setting circuit  3 , the clipper  4  clips the data values of the edge-enhanced image data.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a clipping circuit forperforming wave shaping on a signal fed thereto, and to an imageprocessing device employing such a clipping circuit. The presentinvention relates particularly to a clipping circuit for clippingedge-enhanced image signals, and to an image processing device employingsuch a clipping circuit.

[0003] 2. Description of the Prior Art

[0004] Conventionally, Laplacian processing is used to obtainsatisfactory contrast and sharpness in a reproduced image by making theoutline of a subject clear. Laplacian processing achieves edgeenhancement by adding, to the image signals output from individualpixels to represent a subject of which the outline is to be targeted,signals representing edge components that are calculated by usingLaplacian operator, which is a second-order differential operator. FIGS.8A to 8C show how image signals undergo Laplacian processing. When imagesignals as shown in FIG. 8A are fed in, edge signals representing edgecomponents as shown in FIG. 8B are calculated. Then, as shown in FIG.8C, the edge components represented by those edge signals aresuperimposed on the image signals, so that edge-enhanced image signalsare produced. In this way, it is possible to obtain enhanced sharpnessand satisfactory contrast in a reproduced image.

[0005] As described above, by superimposing edge components representedby edge signals on image signals, it is possible to make changes in datavalues (i.e. changes in signal intensity) at edges sharper, and therebyobtain an edge-enhanced image. However, as FIG. 8C clearly shows, as aresult of Laplacian processing, in the image signals that give the datavalues at edges, preshoots occur in low-data-value portions, andovershoots occur in high-data-value portions. This causes ringing atedges in the reproduced image, and thereby makes the reproduced imageappear unnatural.

SUMMARY OF THE INVENTION

[0006] An object of the present invention is to provide a clippingcircuit that can clip image signals that are fed thereto to enhance thesharpness of the image they represent without causing ringing in thereproduced image, and to provide an image processing device employingsuch a clipping circuit.

[0007] To achieve the above object, according to one aspect of thepresent invention, a clipping circuit is provided with: a clipper forclipping a target signal fed thereto within a range of data values setfor the target signal; a subtractor for subtracting, from the data valueof the target signal to be clipped by the clipper, each of the datavalues of adjacent signals located a predetermined interval away fromthe target signal before and after the target signal; a minimum valuesetter for setting, as the minimum value of the range of data values forthe target signal, the data value of one of the two adjacent signalsthat, when subtracted from the data value of the target signal, yields adifference greater than a first threshold value; and a maximum valuesetter for setting, as the maximum value of the range of data values forthe target signal, the data value of one of the two adjacent signalsthat, when subtracted from the data value of the target signal, yields adifference smaller than a second threshold value. Here, when the datavalue of the target signal fed to the clipper falls within the range ofdata values set for the target signal by the maximum value and minimumvalue setters, the target signal is output intact; when the data valueof the target signal fed to the clipper is smaller than the minimumvalue, the target signal is output after being clipped at the minimumvalue; and, when the data value of the target signal fed to the clipperis greater than the maximum value, the target signal is output afterbeing clipped at the maximum value.

[0008] According to another aspect of the invention, an image processingdevice is provided with: an edge detecting circuit for performingsecond-order differentiation on image signals fed thereto to detect edgecomponents and output edge signals representing the edge components; anedge enhancement circuit for superimposing the edge signals on the imagesignals to perform edge enhancement on the image signals; a rangesetting circuit for comparing, for each of the image signals obtainedfrom individual pixels, the data value of a target image signal obtainedfrom the pixel targeted by the edge enhancement with each of the datavalues of two adjacent image signals obtained from the pixels adjacentto the targeted pixel to set a range of data values in which the datavalue of the target image signal is allowed to vary by setting as themaximum value the data value of one of the two adjacent image signalswhose data value is greater than the data value of the target imagesignal and setting as the minimum value the data value of one of the twoadjacent image signals whose data value is smaller than the data valueof the target image signal; and a clipper for making the data value ofthe target image signal equal to the maximum or minimum value of therange set by the range setting circuit when the data value of the targetimage signal output from the edge enhancement circuit is greater thanthe maximum value or smaller than the minimum value, respectively, ofthe range set by the range setting circuit and for otherwise leavingintact the data value of the target image signal as obtained after theedge enhancement.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] This and other objects and features of the present invention willbecome clear from the following description, taken in conjunction withthe preferred embodiments with reference to the accompanying drawings inwhich:

[0010]FIG. 1 is a block diagram showing the internal configuration of animage processing device embodying the invention;

[0011]FIG. 2 is a block diagram showing the internal configuration ofthe edge detecting circuit;

[0012]FIG. 3 is a block diagram showing the internal configuration ofthe clipper;

[0013]FIG. 4 is a flow chart showing the operation of the range settingcircuit in the image processing device of a first embodiment of theinvention;

[0014]FIG. 5 is a block diagram showing the internal configuration ofthe range setting circuit in the image processing device of a secondembodiment of the invention;

[0015]FIG. 6 is a diagram showing examples of image signals fed to theimage processing device;

[0016]FIGS. 7A to 7D are time charts of relevant signals, illustratingthe operation of an image processing device embodying the invention; and

[0017]FIGS. 8A to 8C are time charts of relevant signals, illustratingthe operation of a conventional image processing device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Overall Configuration of an Image Processing Device

[0019] First, the overall configuration of an image processing deviceembodying the invention will be described with reference to thedrawings. FIG. 1 is a block diagram showing the internal configurationof an image processing device embodying the invention.

[0020] The image processing device shown in FIG. 1 has an edge detectingcircuit 1 that outputs edge signals by detecting edge components inimage signals fed in via an input terminal IN, an adder circuit 2 thatsuperimposes the edge signals produced by the edge detecting circuit 1on the image signals fed in via the input terminal IN, a range settingcircuit 3 that sets the minimum and maximum values of the range in whichthe image signal currently being fed in is allowed to vary on the basisof the image signals output from a plurality of pixels preceding andfollowing the pixel from which the image signal is currently being fedin, and a clipper 4 to which the range setting circuit 3 feeds theminimum and maximum values of the range in which the image signalcurrently being fed in is allowed to vary.

[0021] In the image processing device configured as described above,when image signals are fed in via the input terminal IN, the edgedetecting circuit 1 performs second-order differentiation on the imagesignals to detect edge components present therein, and outputs the thusdetected edge components as edge signals. The internal configuration ofthis edge detecting circuit 1, which produces edge signals in this way,is shown in FIG. 2.

[0022] The edge detecting circuit 1 shown in FIG. 2 has a flip-flop FF1to which the image signals are fed from the input terminal IN, aflip-flop FF2 to which the image signals output from the flip-flop FF1are fed, a multiplier circuit 11 that multiplies the image signalsoutput from the flip-flop FF2 by −¼, a multiplier circuit 12 thatmultiplies the image signals output from the flip-flop FF1 by ½, amultiplier circuit 13 that multiplies the image signals fed in via theinput terminal IN by −¼, an adder circuit 14 that adds together theoutputs from the multiplier circuits 11, 12, and 13, and an amplifier 15that amplifies the output from the adder circuit 14 by a factor of α.

[0023] In the edge detecting circuit 1 configured as described above,when the image signals G1, G2, and G3 output from adjacent pixels arefed in via the input terminal IN consecutively, in the order G1, G2, andG3, the image signal GI is stored in the flip-flop FF2, and the imagesignal G2 is stored in the flip-flop FF1. Then, the image signals G1,G2, and G3 are fed to the multiplier circuits 11, 12, and 13,respectively. Let the data values of the image signals G1, G2, and G3 beg1, g2, and g3, respectively. Then, the multiplier circuits 11, 12, and13 output data values −¼×g1, ½×g2, and −¼×g3, respectively. These datavalues are then added together by the adder circuit 14, and theresulting sum is then amplified by a factor of α by the amplifier 15.Thus, a data value α×(−¼×g1+½×g2−¼×g3) is fed to the adder circuit 2.

[0024] The data value output from the adder circuit 14 can be expressedas [(g2−g1) (g3−g2)]/4, which is the result of second-orderdifferentiation performed for the pixel that has produced the imagesignal G2. Thus, in the edge detecting circuit 1, when the image signalG3 is fed in, second-order differentiation is performed for the imagesignal G2 to detect an edge component, which is then fed out, as an edgesignal, through the amplifier 15.

[0025] The image signals fed in via the input terminal IN are also fedto the range setting circuit 3. The range setting circuit 3 compareseach of the image signals fed thereto from the individual pixels withthe image signals output from a plurality of pixels preceding andfollowing that pixel, and sets the range in which the data value of thatpixel is allowed to vary. Here, when the image signal G3 is fed to theedge detecting circuit 1 as described above, the range setting circuit 3sets the range in which the data value of the image signal G2 is allowedto vary, calculates the minimum value gmi2 and the maximum value gma2 ofthat range, and feeds those values to the clipper 4. To permit the edgedetecting circuit 1 and the range setting circuit 3 to process the sameimage signal at a given time in this way, a delay circuit or the like isprovided within the edge detecting circuit 1 for timing adjustment. Therange setting circuit 3 will be described in detail later.

[0026] The adder circuit 2 adds the edge signals produced by the edgedetecting circuit 1 to the image signals fed in via the input terminalIN. Here, when the image signal G3 is fed to the edge detecting circuit1 as described above, the edge component for the image signal G2 is fedto the adder circuit 2. Thus, to permit the adder circuit 2 to add theedge component produced for the image signal G2 by the edge detectingcircuit 1 to the image signal G2, a delay circuit or the like isprovided within the adder circuit 2 for timing adjustment. As a resultof the adder circuit 2 operating in this way, the image signal G2, afterbeing processed by the adder circuit 2, has a data valueg2−α×(¼×g1−½×g2+¼×g3).

[0027] The image signals thus having their edge components added theretoby the adder circuit 2, i.e. the image signals thereby edge-enhanced,are then fed to the clipper 4. The clipper 4 compares the data value ofeach image signal with the minimum and maximum values fed thereto fromthe range setting circuit 3 to check whether or not the data value ofthat image signal falls within the range in which it is allowed to varyas set by the range setting circuit 3. Specifically, when the data valueof the image signal G2, i.e. g2−α×(¼×g1−½×g2+¼×g3) (hereinafter letg2a=g2−α×(¼×g1−½×g2+¼×g3)), is fed from the adder circuit 2 to theclipper 4 as described above, this data value is compared with theminimum and maximum values gmi2 and gma2 of the range in which the datavalue of the image signal G2 is allowed to vary as set by the rangesetting circuit 3.

[0028] According to the value g2a of the image signal G2 fed from theadder circuit 2 to the clipper 4, the clipper 4, if g2a<gmi2, clips thedata value g2a of the image signal G2 at gmi2 and, if g2a>gma2, clips itat gma2. On the other hand, if gmi2≦g2a≦gma2, the clipper 4 leaves thedata value g2a intact as the data value of the image signal G2. Theinternal configuration of this clipper 4, which sets the data value ofeach image signal according to the range in which it is allowed to varyas set by the range setting circuit 3 in this way, is shown in FIG. 3.

[0029] The clipper 4 shown in FIG. 3 is composed of a selecting circuit4 a that receives, on one hand, an image signal having its edge signaladded thereto from the adder circuit 2 and, on the other hand, themaximum value from the range setting circuit 3 and a selecting circuit 4b that receives, on one hand, the output from the selecting circuit 4 aand, on the other hand, the minimum value from the range setting circuit3. In the clipper 4 configured as described above, the selecting circuit4 a chooses, between the two signals fed thereto, the one having thelower data value, and the selecting circuit 4 b chooses, between the twosignals fed thereto, the one having the higher data value. In theselecting circuits 4 a and 4 b, if the two signals fed thereto haveequal data values, the selecting circuit 4 a chooses the output from theadder circuit 2, and the selecting circuit 4 b chooses the output fromthe selecting circuit 4 a.

[0030] Specifically, when the data value g2a of the image signal G2 isoutput from the adder circuit 2 as described above, first, the selectingcircuit 4 a compares this data value g2a with the maximum value gma2,and outputs the lower data value as the data value of the image signalG2 to the selecting circuit 4 b. Thus, either the data value gma2, ifg2a>gma2, or the data value g2a, if g2a≦gma2, is chosen and output asthe data value of the image signal G2. To permit the selecting circuit 4a to process simultaneously the image signal G2 output from the addercircuit 2 and the maximum value of the range in which the data value ofthe image signal G2 is allowed to vary, a delay circuit or the like isprovided within the selecting circuit 4 a for timing adjustment.

[0031] Let the data value chosen by the selecting circuit 4 a be g2b.Next, the selecting circuit 4 b compares this data value g2b with theminimum value gmi2, and outputs the higher data value as the data valueof the image signal G2 to an output terminal OUT. Thus, either the datavalue gmi2, if g2b<gmi2, or the data value g2b, if g2b≧gmi2, is chosenand output as the data value of the image signal G2 to the outputterminal OUT. To permit the selecting circuit 4 b to processsimultaneously the image signal G2 output from the selecting circuit 4 aand the minimum value of the range in which the data value of the imagesignal G2 is allowed to vary, a delay circuit or the like is providedwithin the selecting circuit 4 b for timing adjustment.

[0032] The internal configuration of the image processing devicedescribed above is common to both of the embodiments described below.Therefore, in the following descriptions of the embodiments, only therange setting circuit, which is configured differently between theembodiments, will be explained, and the explanations of the other blockswill not be repeated.

[0033] First Embodiment

[0034] A first embodiment of the invention will be describe below withreference to the drawings. FIG. 4 is a flow chart showing the operationof the range setting circuit in the image processing device of thisembodiment.

[0035] Suppose that, now, an image signal Gb is currently being targetedas the one for which the range in which its data value (called thetarget data value) is allowed to vary is about to be set, and that theimage signal Gb is fed in immediately after an image signal Ga andimmediately before an image signal Gc. In this case, it is when theimage signal Gc is fed to the range setting circuit 3 (FIG. 1) that therange in which the data value of the image signal Gb is allowed to varyis set. Let the data values of the image signals Ga, Gb, and Gc be ga,gb, and gc, respectively.

[0036] Thus, when the image signal Gc is fed in via the input terminalIN (FIG. 1), first, the difference |ga−gb| between the data values ofthe image signals Ga and Gb is compared with a predetermined thresholdvalue TH (STEP 1). If |ga−gb|≦TH, the data value gb of the image signalGb is chosen as a candidate for the maximum or minimum value thatdetermines the range in which the target data value is allowed to vary(STEP 2). By contrast, if |ga−gb|>TH, the data value ga of the imagesignal Ga is chosen as a candidate for the maximum or minimum value thatdetermines the range in which the target data value is allowed to vary(STEP 3).

[0037] When the flow proceeds from STEP 1 to STEP2, then the difference|gb−gc| between the data values of the image signals Gb and Gc iscompared with the predetermined threshold value TH (STEP 4). If|gb−gc|≦TH, the data value gb of the image signal Gb is chosen as acandidate for the maximum or minimum value that determines the range inwhich the target data value is allowed to vary (STEP 5). In this case,as a candidate for the maximum or minimum value, the data value gb ofthe image signal Gb has been chosen in both STEP 2 and STEP 5, andtherefore this data value gb is set as the maximum and minimum values.Then, the flow proceeds to STEP 13, where the data value gb thus set asthe maximum and minimum values is fed to the clipper 4 (FIG. 1).

[0038] If, in STEP 4, |gb−gc|>TH, the data value gc of the image signalGc is chosen as a candidate for the maximum or minimum value thatdetermines the range in which the target data value is allowed to vary(STEP 6). In this case, as a candidate for the maximum or minimum valuethat determines the range in which the target data value is allowed tovary, the data value gb has been chosen in STEP 2, and the data value gchas been chosen in STEP 6; thus, of these data values gb and gc, the onehaving the higher data value is set as the maximum value, and the onehaving the lower data value is set as the minimum value (STEP 12). Themaximum and minimum values thus set are then fed to the clipper 4(FIG. 1) (STEP 13).

[0039] On the other hand, when the flow proceeds from STEP 1 to STEP3,then the difference |gb−gc| between the data values of the image signalsGb and Gc is compared with the predetermined threshold value TH (STEP7). If |gb−gc|≦TH, the data value gb of the image signal Gb is chosen asa candidate for the maximum or minimum value that determines the rangein which the target data value is allowed to vary (STEP 8). In thiscase, as a candidate for the maximum or minimum value that determinesthe range in which the target data value is allowed to vary, the datavalue ga has been chosen in STEP 3, and the data value gb has beenchosen in STEP 8; thus, of these data values ga and gb, the one havingthe higher data value is set as the maximum value, and the one havingthe lower data value is set as the minimum value (STEP 12). The maximumand minimum values thus set are then fed to the clipper 4 (FIG. 1) (STEP13).

[0040] If, in STEP 7, |gb−gc|>TH, then whether or not the data valuesga, gb, and gc of the image signals Ga, Gb, and Gc satisfy the conditionga<gb<gc or gc<gb<ga is checked (STEP 9). If the data values ga, gb, andgc satisfy either of the conditions, the data value gc of the imagesignal Gc is chosen as a candidate for the maximum or minimum value thatdetermines the range in which the target data value is allowed to vary(STEP 10). In this case, as a candidate for the maximum or minimum valuethat determines the range in which the target data value is allowed tovary, the data value ga has been chosen in STEP 3, and the data value gchas been chosen in STEP 10; thus, of these data values ga and gc, theone having the higher data value is set as the maximum value, and theone having the lower data value is set as the minimum value (STEP 12).The maximum and minimum values thus set are then fed to the clipper 4(FIG. 1) (STEP 13).

[0041] If, in STEP 9, the data values ga, gb, and gc satisfy neither ofthe aforementioned conditions, the data value gb is chosen as both themaximum and minimum values that determine the range in which the targetdata value is allowed to vary (STEP 11). The maximum and minimum valuesthus set are then fed to the clipper 4 (FIG. 1) (STEP 13).

[0042] When the maximum and minimum values thus set as a result of therange setting circuit 3 operating as described above are fed to theclipper 4, the clipper 4 then determines the data value of the imagesignal Gb on the basis of those maximum and minimum values. It is to benoted that, in STEPs 1, 4, and 7, when the difference between two imagesignals is compared with the threshold value TH, the noise componentsthat are superimposed on those image signals and of which the levels arelower than the threshold value TH are absorbed. This helps reduce theeffect of such noise components on the operation of the range settingcircuit 3.

[0043] Second Embodiment

[0044] A second embodiment of the invention will be describe below withreference to the drawings. FIG. 5 is a block diagram showing theinternal configuration of the range setting circuit in the imageprocessing device of this embodiment.

[0045] The range setting circuit 3 shown in FIG. 5 is composed offlip-flops FFa, FFb, FFc, FFd, FFe, and FFf for storing image signals,subtractor circuits 31 a, 31 b, 31 c, 31 d, 31 e, and 31 f thatcalculate differences between every two consecutive image signals,comparators 32 a, 32 b, 32 c, 32 d, 32 e, and 32 f that compare thedifferences between image signals output from the subtractor circuits 31a, 31 b, 31 c, 31 d, 31 e, and 31 f with threshold values ±TH, and adecoder 33 that sets the maximum or minimum value that determines therange in which the target data value is allowed to vary.

[0046] In the range setting circuit 3 configured as described above, theimage signal fed in via the input terminal IN is fed to the flip-flopFFa, to the subtractor circuit 31 a, and to the decoder 33. The imagesignal output from the flip-flop FFa is fed to the flip-flop FFb, to thesubtractor circuits 31 a and 31 b, and to the decoder 33. The imagesignal output from the flip-flop FFb is fed to the flip-flop FFc, to thesubtractor circuits 31 b and 31 c, and to the decoder 33. The imagesignal output from the flip-flop FFc is fed to the flip-flop FFd, to thesubtractor circuits 31 c and 31 d, and to the decoder 33. The imagesignal output from the flip-flop FFd is fed to the flip-flop FFe, to thesubtractor circuits 31 d and 31 e, and to the decoder 33. The imagesignal output from the flip-flop FFe is fed to the flip-flop FFf, to thesubtractor circuits 31 e and 31 f, and to the decoder 33. The imagesignal output from the flip-flop FFf is fed to the subtractor circuit 31f and to the decoder 33.

[0047] The subtractor circuit 31 a subtracts the image signal outputfrom the flip-flop FFa from the image signal fed in via the inputterminal IN. The subtractor circuit 31 b subtracts the image signaloutput from the flip-flop FFb from the image signal output from theflip-flop FFa. The subtractor circuit 31 c subtracts the image signaloutput from the flip-flop FFc from the image signal output from theflip-flop FFb. The subtractor circuit 31 d subtracts the image signaloutput from the flip-flop FFc from the image signal output from theflip-flop FFd. The subtractor circuit 31 e subtracts the image signaloutput from the flip-flop FFd from the image signal output from theflip-flop FFe. The subtractor circuit 31 f subtracts the image signaloutput from the flip-flop FFe from the image signal output from theflip-flop FFf

[0048] The results of the subtraction performed by the subtractorcircuits 31 a to 31 f are fed to the comparators 32 a to 32 f,respectively, so as to be compared with the threshold values ±TH. Thecomparators 32 a to 32 f individually output, to the decoder 33, a“positive” sign if the subtraction result from the corresponding one ofthe subtractor circuits 31 a to 31 f is greater than +TH, a “negative”sign if the subtraction result from the corresponding one of thesubtractor circuits 31 a to 31 f is smaller than −TH, or a “zero” signif the subtraction result from the corresponding one of the subtractorcircuits 31 a to 31 f is equal to or greater than −TH or equal to orsmaller than +TH.

[0049] On the basis of these signs fed from the comparators 32 a to 32f, the decoder 33 selects the maximum and minimum data values from amongthe data values of the seven image signals fed in via the input terminalIN and output from the flip-flops FFa to FFf, and feeds the thusselected maximum and minimum data values to the clipper 4 (FIG. 1).

[0050] Now, how the range setting circuit 3 configured as describedabove operates will be described. Suppose that image signals Gc2, Gb2,Ga2, Gx, Ga1, Gb1, and Gc1 are fed in in the order Gc1, Gb1, Ga1, Gx,Ga2, Gb2, and Gc2, and that the image signal Gx is currently beingtargeted as the one for which the maximum and minimum values thatdetermine the range in which its data value is allowed to vary is aboutto be set. Let the data values of the image signals Gc2, Gb2, Ga2, Gx,Ga1, Gb1, and Gc1 be gc2, gb2, ga2, gx, ga1, gb1, and gc1, respectively.

[0051] When the image signals are fed in in the order Gc1, Gb1, Ga1, Gx,Ga2, Gb2, and Gc2, the data values gb2, ga2, gx, ga1, gb1, and gc1 ofthe image signals Gb2, Ga2, Gx, Ga1, Gb1, and Gc1 are stored in theflip-flops FFa, FFb, FFc, FFd, FFe, and FFf, respectively. When theimage signal Gc2 is fed in via the input terminal IN, the flip-flopsFFa, FFb, FFc, FFd, FFe, and FFf output the image signals Gb2, Ga2, Gx,Ga1, Gb1, and Gc1, and the image signals Gc2, Gb2, Ga2, Gx, Ga1, and Gb1are fed to the flip-flops FFa, FFb, FFc, FFd, FFe, and FFf

[0052] At this point, the subtractor circuits 31 a, 31 b, 31 c, 31 d, 31e, and 31 f individually perform subtraction and output their respectivesubtraction results (gc2−gb2), (gb2−ga2), (ga2−gx), (ga1−gx), (gb1−ga1),and (gc1−gb1) to the comparators 32 a, 32 b, 32 c, 32 d, 32 e, and 32 f.Then, the comparators 32 a, 32 b, 32 c, 32 d, 32 e, and 32 f comparethese subtraction results (gc2−gb2), (gb2−ga2), (ga2−gx), (ga1−gx),(gb1−ga1), and (gc1−gb1) with the threshold values ±TH, and output theirrespective comparison results, as signs da, db, dc, dd, de, and df, tothe decoder 33. These signs da, db, dc, dd, de, and df each representone of the three aforementioned states, namely “positive,” “negative,”and “zero.” In the following descriptions, the states “positive,”“negative,” and “zero” are represented simply by “+,” “−,” and “0,”respectively.

[0053] 1. When (dc, dd)=(+, +), (dc, d)=(−, −), or (dc, dd)=(0, 0),

[0054] When both of the signs dc and dd are “+,” “−,” or “0,”irrespective of the values of the other signs da, db, de, and df, thedecoder 33 selects the data value gx of the image signal Gx as themaximum and minimum values, and outputs these maximum and minimum valuesto the clipper 4.

[0055] 2. When (dc, dd) (0, +)

[0056] When the sign dc is “0” and the sign dd is “+,” irrespective ofthe values of the signs da and db, the decoder 33 selects the data valuegx of the image signal Gx as the minimum value, and outputs this minimumvalue to the clipper 4. On the other hand, as the maximum value, thedecoder 33 selects and outputs different data values according towhether (a) de=0 or −, (b) (de, df)=(+, 0) or (+, −), or (c) (de, df)(+, +) as described below.

[0057] (a) When de=0 or −

[0058] Irrespective of the value of the sign df, the data value ga1 ofthe image signal Ga1 is selected as the maximum value and is output tothe clipper 4.

[0059] (b) When (de, df)=(+, 0) or (+, −)

[0060] The data value gb1 of the image signal Gb1 is selected as themaximum value and is output to the clipper 4.

[0061] (c) When (de, df)=(+, +)

[0062] The data value gc1 of the image signal Gc1 is selected as themaximum value and is output to the clipper 4.

[0063] 3. When (dc, dd)=(0, −)

[0064] When the sign dc is “0” and the sign ad is “−,” irrespective ofthe values of the signs da and db, the decoder 33 selects the data valuegx of the image signal Gx as the maximum value, and outputs this maximumvalue to the clipper 4. On the other hand, as the minimum value, thedecoder 33 selects and outputs different data values according towhether (a) de=0 or +, (b) (de, df)=(−, 0) or (−, +), or (c) (de,df)=(−, −) as described below.

[0065] (a) When de=or +

[0066] Irrespective of the value of the sign df, the data value ga1 ofthe image signal Ga1 is selected as the minimum value and is output tothe clipper 4.

[0067] (b) When (de, df)=(−, 0) or (−, +)

[0068] The data value gb1 of the image signal Gb1 is selected as theminimum value and is output to the clipper 4.

[0069] (c) When (de, df)=(−, −)

[0070] The data value gc1 of the image signal Gc1 is selected as theminimum value and is output to the clipper 4.

[0071] 4. When (dc, dd)=(+, 0)

[0072] When the sign dc is “+” and the sign ad is “0,” irrespective ofthe values of the signs de and df, the decoder 33 selects the data valuegx of the image signal Gx as the minimum value, and outputs this minimumvalue to the clipper 4. On the other hand, as the maximum value, thedecoder 33 selects and outputs different data values according towhether (a) db=0 or −, (b) (da, db)=(0, +) or (−, +), or (c) (da,db)=(+, +) as described below.

[0073] (a) When db=0 or −

[0074] Irrespective of the value of the sign da, the data value ga2 ofthe image signal Ga2 is selected as the maximum value and is output tothe clipper 4.

[0075] (b) When (da, db)=(0, +) or (−, +)

[0076] The data value gb2 of the image signal Gb2 is selected as themaximum value and is output to the clipper 4.

[0077] (c) When (da, db)=(+, +)

[0078] The data value gc2 of the image signal Gc2 is selected as themaximum value and is output to the clipper 4.

[0079] 5. When (dc, dd)=(−, 0)

[0080] When the sign dc is “−” and the sign dd is “0,” irrespective ofthe values of the signs de and df, the decoder 33 selects the data valuegx of the image signal Gx as the maximum value, and outputs this maximumvalue to the clipper 4. On the other hand, as the minimum value, thedecoder 33 selects and outputs different data values according towhether (a) db=0 or +, (b) (da, db)=(0, −) or (+, −), or (c) (da,db)=(−, −) as described below.

[0081] (a) When db=0 or +

[0082] Irrespective of the value of the sign da, the data value ga2 ofthe image signal Ga2 is selected as the minimum value and is output tothe clipper 4.

[0083] (b) When (da, db)=(0, −) or (+, −)

[0084] The data value gb2 of the image signal Gb2 is selected as theminimum value and is output to the clipper 4.

[0085] (c) When (da, db)=(−, −)

[0086] The data value gc2 of the image signal Gc2 is selected as theminimum value and is output to the clipper 4.

[0087] 6. When (dc, dd)=(−, +)

[0088] As the minimum value, the decoder 33 selects and outputsdifferent data values according to whether (a) db=0 or +, (b) (da, db)=(0, −) or (+, −, or (c) (da, db) =(−, −) as described below.

[0089] (a) When db=0 or +

[0090] Irrespective of the value of the sign da, the data value ga2 ofthe image signal Ga2 is selected as the minimum value and is output tothe clipper 4.

[0091] (b) When (da, db)=(0, −) or (+, −)

[0092] The data value gb2 of the image signal Gb2 is selected as theminimum value and is output to the clipper 4.

[0093] (c) When (da, db)=(−, −)

[0094] The data value gc2 of the image signal Gc2 is selected as theminimum value and is output to the clipper 4.

[0095] On the other hand, as the maximum value, the decoder 33 selectsand outputs different data values according to whether (a) de=0 or −,(b) (de, df)=(+, 0) or (+, −), or (c) (de, df)=(+, +) as describedbelow.

[0096] (a) When de=0 or −

[0097] Irrespective of the value of the sign df, the data value ga1 ofthe image signal Ga1 is selected as the maximum value and is output tothe clipper 4.

[0098] (b) When (de, df)=(+, 0) or (+, −)

[0099] The data value gb1 of the image signal Gb1 is selected as themaximum value and is output to the clipper 4.

[0100] (c) When (de, df)=(+, +)

[0101] The data value gc1 of the image signal Gc1 is selected as themaximum value and is output to the clipper 4.

[0102] 7. When (de, dd)=(+, −)

[0103] As the minimum value, the decoder 33 selects and outputsdifferent data values according to whether (a) de=0 or +, (b) (de,df)=(−, 0) or (−, +),or (c) (de, df)=(−, −) as described below.

[0104] (a) When de=0 or +

[0105] Irrespective of the value of the sign df, the data value ga1 ofthe image signal Ga1 is selected as the minimum value and is output tothe clipper 4.

[0106] (b) When (de, df)=(−, 0) or (−, +)

[0107] The data value gb1 of the image signal Gb1 is selected as theminimum value and is output to the clipper 4.

[0108] (c) When (de, df)=(−, −)

[0109] The data value gc1 of the image signal Gc1 is selected as theminimum value and is output to the clipper 4.

[0110] On the other hand, as the maximum value, the decoder 33 selectsand outputs different data values according to whether (a) db=0 or −,(b) (da, db)=(0, +) or (−, +), or (c) (da, db)=(+, +) as describedbelow.

[0111] (a) When db=0 or −

[0112] Irrespective of the value of the sign da, the data value ga2 ofthe image signal Ga2 is selected as the maximum value and is output tothe clipper 4.

[0113] (b) When (da, db)=(0, +) or (−, +)

[0114] The data value gb2 of the image signal Gb2 is selected as themaximum value and is output to the clipper 4.

[0115] (c) When (da, db)=(+, +)

[0116] The data value gc2 of the image signal Gc2 is selected as themaximum value and is output to the clipper 4.

[0117] More specifically, when the image signals Gc2, Gb2, Ga2, Gx, Ga1,Gb1, and Gc1 have signal values as shown in FIG. 6, the range settingcircuit 3 operates as follows. Suppose, here also, that the imagesignals shown in FIG. 6 are fed in in the order Gc1, Gb1, Ga1, Gx, Ga2,Gb2, and Gc2, and that the image signal Gx is currently being targetedas the one for which the maximum and minimum values that determine therange in which its data value is allowed to vary is about to be set.

[0118] As described earlier, when the image signals Gc2, Gb2, Ga2, Gx,Ga1, Gb1, and Gc1 are output from the flip-flops FFa to FFf and fed invia the input terminal IN, the subtractor circuits 31 a to 31 findividually perform subtraction and output their respective subtractionresults (gc2−gb2), (gb2−ga2), (ga2−gx), (ga1−gx), (gb1−ga1), and(gc1−gb1) to the comparators 32 a, 32 b, 32 c, 32 d, 32 e, and 32 fHere, assume that, as shown in FIG. 6, the data values of the imagesignals satisfy the following relations: (gc2−gb2)<−TH, (gb2−ga2)<−TH,(ga2−gx)<−TH, (ga1−gx)>TH, −TH≦(gb1−ga1)≦TH, and −TH≦(gc1−gb1)≦TH.

[0119] At this point, the signs da, db, dc, ad, de, and df output fromthe comparator 32 a to 32 f are: (da, db, dc, ad, de, df)=(−, −, −, +,0, 0). Fed with the signs da to df with these values, the decoder 33selects the data value gc2 of the image signal Gc2 as the minimum valueand the data value ga1 of the image signal Ga1 as the maximum value, andoutputs these minimum and maximum values to the clipper 4.

[0120] In this embodiment, the range setting circuit 3 sets the maximumand minimum values of the data value of the image signal to be correctedby the clipper 4 on the basis of the relationship of that image signalwith three image signals preceding and three image signals followingthat image signal. However, the image signals so referred to are notlimited to three image signals preceding and three image signalsfollowing the target image signal. The range setting circuit 3 may beconfigured in any other manner than is specifically shown in FIG. 5, aslong as it operates in the same manner as in this embodiment.

[0121] In the image processing devices of the first and secondembodiments described above, as a result of the operation of the rangesetting circuit 3, image signals are processed as follows. When imagesignals as shown in FIG. 7A are fed in, first, the edge detectingcircuit 1 performs second-order differentiation on those image signals(FIG. 7A) to produce edge signals as shown in FIG. 7B. Then, the addercircuit 2 adds together the image signals (FIG. 7A) and the edge signals(FIG. 7B) to produce image signals as shown in FIG. 7C that have suchtransient characteristics as to include overshoots and preshootsoccurring in the portions thereof corresponding to edges.

[0122] Then, the clipper 4 clips these image signals (FIG. 7C) withinthe range set by the range setting circuit 3. As a result, image signalsas shown in FIG. 7D are output that have been edge-enhanced and thathave their transient characteristics improved.

[0123] According to the present invention, the range in which the datavalue of a target signal is to be clipped by a clipping circuit is seton the basis of the data values obtained by comparing the target signalwith the signals nearest thereto, then those nearest signals with thesecond nearest, and so forth, i.e. by comparing pairs of consecutiveimage signals in order of proximity to the target image signal. Thus,for example, in a case where the signal so clipped by the clippingcircuit is an image signal, and the image signal is clipped after beingedge-enhanced, i.e. after having an edge signal, as obtained byperforming second-order differentiation on the image signal, superposedthereon, it is possible to eliminate overshoots and undershootsresulting from edge enhancement. Accordingly, image signals, whenprocessed by an image processing device provided with a clipping circuitoperating in this way, have their transient characteristics improved aswell as being edge-enhanced. In an image obtained by reproducing imagesignals obtained in such a way, ringing is satisfactorily suppressed.

What is claimed is:
 1. A clipping circuit comprising: a clipper forclipping a target signal fed thereto within a range of data values setfor the target signal; a subtractor for subtracting, from a data valueof the target signal to be clipped by the clipper, each of data valuesof adjacent signals located a predetermined interval away from thetarget signal before and after the target signal; a minimum value setterfor setting, as a minimum value of the range of data values for thetarget signal, the data value of one of the two adjacent signals that,when subtracted from the data value of the target signal, yields adifference greater than a first threshold value; and a maximum valuesetter for setting, as a maximum value of the range of data values forthe target signal, the data value of one of the two adjacent signalsthat, when subtracted from the data value of the target signal, yields adifference smaller than a second threshold value, wherein, when the datavalue of the target signal fed to the clipper falls within the range ofdata values set for the target signal by the maximum value and minimumvalue setters, the target signal is output intact, when the data valueof the target signal fed to the clipper is smaller than the minimumvalue, the target signal is output after being clipped at the minimumvalue, and, when the data value of the target signal fed to the clipperis greater than the maximum value, the target signal is output afterbeing clipped at the maximum value.
 2. A clipping circuit comprising: aclipper for clipping a target signal fed thereto within a range of datavalues set for the target signal; a first subtractor for calculating,for the target signal to be clipped by the clipper and a plurality ofsignals located at predetermined intervals from one another before andafter the target signal, a difference between data values of every twoadjacent signals by subtracting the data value of one of those twosignals farther from the target signal from the data value of one ofthose two signals nearer to the target signal, in order of proximity tothe target signal; a second subtractor for calculating a differencebetween the data value of each of the two adjacent signals nearest tothe target signal and the data value of the target signal by subtractingthe data value of that adjacent signal from the data value of the targetsignal; a minimum value setter for setting, as a minimum value of therange of data values for the target signal, the data value of whicheveris nearer to the target signal out of two signals whose data value, whenprocessed by the first subtractor, yields a difference smaller than afirst threshold value and that are located farther than one of theadjacent signals whose data value, when processed by the secondsubtractor, yields a difference greater than the first threshold value;and a maximum value setter for setting, as a maximum value of the rangeof data values for the target signal, the data value of whichever isnearer to the target signal out of two signals whose data value, whenprocessed by the first subtractor, yields a difference greater than asecond threshold value and that are located farther than one of theadjacent signals whose data value, when processed by the secondsubtractor, yields a difference smaller than the first threshold value,wherein, when the data value of the target signal fed to the clipperfalls within the range of data values set for the target signal by themaximum value and minimum value setters, the target signal is outputintact, when the data value of the target signal fed to the clipper issmaller than the minimum value, the target signal is output after beingclipped at the minimum value, and, when the data value of the targetsignal fed to the clipper is greater than the maximum value, the targetsignal is output after being clipped at the maximum value.
 3. An imageprocessing device comprising: an edge detecting circuit for performingsecond-order differentiation on image signals fed thereto to detect edgecomponents and output edge signals representing the edge components; anedge enhancement circuit for superimposing the edge signals on the imagesignals to perform edge enhancement on the image signals; a rangesetting circuit for comparing, for each of image signals obtained fromindividual pixels, a data value of a target image signal obtained from apixel targeted by the edge enhancement with each of data values of twoadjacent image signals obtained from pixels adjacent to the targetedpixel to set a range of data values in which the data value of thetarget image signal is allowed to vary by setting as a maximum value thedata value of one of the two adjacent image signals whose data value isgreater than the data value of the target image signal and setting as aminimum value the data value of one of the two adjacent image signalswhose data value is smaller than the data value of the target imagesignal; and a clipper for making the data value of the target imagesignal equal to the maximum or minimum value of the range set by therange setting circuit when the data value of the target image signaloutput from the edge enhancement circuit is greater than the maximumvalue or smaller than the minimum value, respectively, of the range setby the range setting circuit, the clipper otherwise leaving intact thedata value of the target image signal as obtained after the edgeenhancement.
 4. An image processing device as claimed in claim 3,wherein the range setting circuit sets, as the maximum value, the datavalue of one of the two adjacent image signals whose data value, whencompared with the data value of the target image signal, yields adifference greater than a predetermined threshold value and whose datavalue is greater than the data value of the target image signal, andsets, as the minimum value, the data value of one of the two adjacentimage signals whose data value, when compared with the data value of thetarget image signal, yields a difference greater than the predeterminedthreshold value and whose data value is smaller than the data value ofthe target image signal.
 5. An image processing device as claimed inclaim 4, wherein, when the difference between the data value of thetarget image signal and the data value of one of the two adjacent imagesignals is smaller than the threshold value, if the range settingcircuit sets the data value of the other of the two adjacent imagesignals as the maximum value, the range setting circuit sets the datavalue of the target image signal as the minimum value, and, if the rangesetting circuit sets the data value of the other of the two adjacentimage signals as the minimum value, the range setting circuit sets thedata value of the target image signal as the maximum value, and when thedifference between the data value of the target image signal and thedata value of either of the two adjacent image signals is smaller thanthe threshold value, the range setting circuit sets the data value ofthe target image signal as both the maximum and minimum values.
 6. Animage processing device as claimed in claim 4, wherein, when the datavalues of both of the two adjacent image signals are greater than thedata value of the target image signal, or when the data values of bothof the two adjacent image signals are smaller than the data value of thetarget image signal, the range setting circuit sets the data value ofthe target image signal as both the maximum and minimum values.
 7. Animage processing device comprising: an edge detecting circuit forperforming second-order differentiation on image signals fed thereto todetect edge components and output edge signals representing the edgecomponents; an edge enhancement circuit for superimposing the edgesignals on the image signals to perform edge enhancement on the imagesignals; a range setting circuit for comparing, for each of imagesignals obtained from individual pixels, data values of a plurality ofconsecutive image signals preceding and following a target image signaltargeted by the edge enhancement to set a range of data values in whichthe data value of the target image signal is allowed to vary; and aclipper for making the data value of the target image signal equal tothe maximum or minimum value of the range set by the range settingcircuit when the data value of the target image signal output from theedge enhancement circuit is greater than the maximum value or smallerthan the minimum value, respectively, of the range set by the rangesetting circuit, the clipper otherwise leaving intact the data value ofthe target image signal as obtained after edge enhancement, wherein therange setting circuit first compares the data value of the target imagesignal obtained from the targeted pixel with each of data values of twoadjacent image signals obtained from pixels adjacent to the targetedpixel to set, as a maximum value setting image signal, one of the twoadjacent image signals whose data value is greater than the data valueof the target image signal and set, as a minimum value setting imagesignal, one of the two adjacent image signals whose data value issmaller than the data value of the target image signal, then comparesthe data value of the maximum value setting image signal with a datavalue of an image signal obtained from a pixel adjacent to the pixelthat outputs the maximum value setting image signal so that, if the datavalue of the maximum value setting image signal is greater, the datavalue of the maximum value setting image signal is kept as the maximumvalue and, if the data value of the maximum value setting image signalis smaller, the maximum value setting image signal is replaced with theimage signal that has been compared with the maximum value setting imagesignal and comparison is continued with a data value of an image signalobtained from a pixel further adjacent, and then similarly compares thedata value of the minimum value setting image signal with a data valueof an image signal obtained from a pixel adjacent to the pixel thatoutputs the minimum value setting image signal so that, if the datavalue of the minimum value setting image signal is smaller, the datavalue of the minimum value setting image signal is kept as the minimumvalue and, if the data value of the minimum value setting image signalis greater, the minimum value setting image signal is replaced with theimage signal that has been compared with the minimum value setting imagesignal and comparison is continued with a data value of an image signalobtained from a pixel further adjacent.
 8. An image processing device asclaimed in claim 7, wherein the range setting circuit sets, as themaximum value, the data value of the maximum value setting image signalas it is when replacement of the maximum value setting image signal hastaken place a predetermined number of times, and sets, as the minimumvalue, the data value of the minimum value setting image signal as it iswhen replacement of the minimum value setting image signal has takenplace a predetermined number of times.
 9. An image processing device asclaimed in claim 7, wherein, if a difference between the data value ofone of the two adjacent image signals and the data value of the targetimage signal is greater than a predetermined threshold value and inaddition the data value of that adjacent image signal is greater thanthe data value of the target image signal, the range setting circuitsets that adjacent image signal as the maximum value setting imagesignal, and, if a difference between the data value of the maximum valuesetting image signal and the data value of the image signal adjacent tothe maximum value setting image signal is greater than the predeterminedthreshold value and in addition the data value of this image signaladjacent to the maximum value setting image signal is greater than thedata value of the maximum value setting image signal, the range settingcircuit performs replacement of the maximum value setting image signal,and similarly if a difference between the data value of one of the twoadjacent image signals and the data value of the target image signal isgreater than the predetermined threshold value and in addition the datavalue of that adjacent image signal is smaller than the data value ofthe target image signal, the range setting circuit sets that adjacentimage signal as the minimum value setting image signal, and, if adifference between the data value of the minimum value setting imagesignal and the data value of the image signal adjacent to the minimumvalue setting image signal is greater than the predetermined thresholdvalue and in addition the data value of this image signal adjacent tothe minimum value setting image signal is smaller than the data value ofthe minimum value setting image signal, the range setting circuitperforms replacement of the minimum value setting image signal.
 10. Animage processing device as claimed in claim 9, wherein, when thedifference between the data value of the target image signal and thedata value of one of the two adjacent image signals is smaller than thethreshold value, if the range setting circuit sets the data value of theother of the two adjacent image signals as the maximum value, the rangesetting circuit sets the data value of the target image signal as theminimum value, and, if the range setting circuit sets the data value ofthe other of the two adjacent image signals as the minimum value, therange setting circuit sets the data value of the target image signal asthe maximum value, and when the difference between the data value of thetarget image signal and the data value of either of the two adjacentimage signals is smaller than the threshold value, the range settingcircuit sets the data value of the target image signal as both themaximum and minimum values.
 11. An image processing device as claimed inclaim 7, wherein, when the data values of both of the two adjacent imagesignals are greater than the data value of the target image signal, orwhen the data values of both of the two adjacent image signals aresmaller than the data value of the target image signal, the rangesetting circuit sets the data value of the target image signal as boththe maximum and minimum values.